Some speech capturing systems require a close-talking microphone located a few inches to the side of a talker's mouth, when the talker is in a noisy environment. However, these microphones are too cumbersome for many applications requiring speech input. There is a need for a speech capturing system that does not require a close-talking microphone.
Other microphones, such as microphone arrays, include signal-processing methods that reduce reverberation and noise. These signal-processing methods need a narrow sensitivity region. FIG. 1 is a block diagram of an example microphone array oriented in three-dimensional space. A sensitivity region (a/k/a pick-up pattern or sensitivity pattern) is an area near the system where speech is picked-up; thus, speech outside the sensitivity region is not adequately captured. FIG. 2 is a graph in polar coordinates showing the sensitivity region of the example microphone array of FIG. 1 of a 1-kHz tone presented to the microphone array at various locations along the x-axis. FIG. 3 is another graph in polar coordinates showing the sensitivity region of the example microphone of FIG. 1 of a 1-kHz tone presented to the microphone array at various locations along the y-axis.
The narrow sensitivity regions required by the signal processing methods are invisible to the eye and often narrower than a talker's normal head movement. One example is a microphone array along the top of a computer monitor with a ±30 degree azimuth sensitivity region. Another example is a microphone in an automobile with a ±15 degree azimuth sensitivity region. Given these narrow sensitivity regions, it is too easy for the talker to unknowingly move their mouth in and out of this region, resulting in captured speech that wavers between audible and inaudible. Yet, if this region is broadened to account for normal head movement, the system's ability to reject noise and reverberation is diminished. There is a need for a speech capturing system that avoids the wavering problem, without broadening the sensitivity region.
Some speech capturing systems attempt to electronically steer a narrow beam to the source of speech based on direction of arrival and tracking schemes. These methods do not work well because they cannot track fast enough and cannot predict movement when the talker pauses without large signal delays. Steering always lags the speech and cannot predict where speech will resume after a silent period. Furthermore, steering done with directional beam formations causes high frequency fluctuations in captured speech. There is a need for a new approach, one that brings the talker to the narrow sensitivity region, rather than reaching out to the talker. There is a need for a way to guide the talker to the narrow sensitivity region and to assure the talker remains in the region, without resorting to steering.